Upon reaching a target, the missile will make attack
rolls against that target and up to three others within
Adjacent range.

Does that mean that the target ship applies PD, but all 4 targets then just resolve what is left of the salvo?

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It seems those don't do much in capital ship battles. We might put a (house) rule that they only account the Defence of the fleet by half(?), for the salvo attack. It will affect squadrons, but not bigger ships. The damage is only 5 anyways.

1.) If the target has other craft that are adjacent to the target of the salvo, up to three targets. Typically would be smallcraft as most other warships would operate at longer range from each other.
2.) If fired at a transiting missile salvo, it applies its own salvo value vs the targeted salvo on a one to one basis.